Throughout this application various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference in this application in order to more fully describe the state of the art to which this invention pertains.
1. Field of the Invention
The present invention relates generally to optical traps. In particular, the invention relates to an apparatus, system and method for applying optical gradient forces to form a plurality of optical traps to manipulate small particles.
2. Discussion of the Related Arts
An optical tweezer is an optical tool which utilizes the gradient forces of a focused beam of light to manipulate particles with dielectric constants higher than the surrounding media. To minimize its energy such particles will move to the region where the electric field is the highest. Stated in terms of momentum, the focused beam of light produces radiation pressure, creating small forces by absorption, reflection, diffraction or refraction of the light by a particle. The forces generated by radiation pressure are almost negligible—a light source, such as a diode-pumped Nd:YAG laser operating at 10 mW, will only produce a few picoNewtons. However, a few picoNewtons of force is sufficient to manipulate small particles.
Other optical tools which can be used to manipulate small particles include, but are not limited to, optical vortices, optical bottles, optical rotators and light cages. An optical vortex, although similar in use to an optical tweezer, operates on an opposite principle. An optical vortex produces a gradient surrounding an area of zero electric field which is useful to manipulate particles with dielectric constants lower than the surrounding media or which are reflective, or other types of particles which are repelled by an optical tweezer. To minimize its energy such a particle will move to the region where the electric field is the lowest, namely the zero electric field area at the focal point of an appropriately shaped laser beam. The optical vortex provides an area of zero electric field much like the hole in a doughnut (toroid). The optical gradient is radial with the highest electric field at the circumference of the doughnut. The optical vortex detains a small particle within the hole of the doughnut. The detention is accomplished by slipping the vortex over the small particle along the line of zero electric field.
The optical bottle differs from an optical vortex in that it has a zero electric field only at the focus and a non-zero electric field at an end of the vortex. An optical bottle may be useful in trapping atoms and nanoclusters which may be too small or too absorptive to trap with an optical vortex or optical tweezers. J. Arlt and M. J. Padgett. “Generation of a beam with a dark focus surrounded by regions of higher intensity: The optical bottle beam,” Opt. Lett. 25, 191-193, 2000.
The optical rotator is a recently described optical tool which provides a pattern of spiral arms which trap objects. Changing the pattern causes the trapped objects to rotate. L. Paterson, M. P. MacDonald, J. Arlt, W. Sibbett, P. E. Bryant, and K. Dholakia, “Controlled rotation of optically trapped microscopic particles,” Science 292, 912-914, 2001. This class of tool may be useful for manipulating non-spherical particles and driving MEMs devices or nano-machinery.
The light cage, described by Neal in U.S. Pat. No. 5,939,716, is loosely, a macroscopic cousin of the optical vortex. A light cage forms a ring of optical vortices to surround a particle too large, too reflective, or with dielectric constants lower than the surrounding media. If the optical vortex is like a doughnut, the light cage is like a jelly-filled doughnut. While the doughnut hole (for the vortex) is an area of zero electric field, the jelly-fill is an area of lowered electric field. In a gross sense, the gradient forces of the plurality of optical tweezers forming the doughnut “push” a particle, with a dielectric constant lower than the surrounding medium, towards the jelly-fill which may also be thought of as the less bright region which lies between the plurality of optical tweezers. However, unlike a vortex, no-zero electric field area is created.
Using a single beam of laser light with a diffractive optical element to form a plurality of diffracted laser beams focused to form an array of optical traps is known in the art. U.S. Pat. No. 6,055,106 issued to Grier and Dufresne describes arrays of optical traps. The patent teaches the use of physical transfer lenses to direct a diffracted laser beam to the back aperture of a focusing lens. Multiple physical lenses are employed to direct and overlay the laser beams at the back aperture of a focusing lens with sufficient overlap to achieve an effective numerical aperture (NA) of at least about 0.8, which, as taught in U.S. Pat. No. 5,079,169 issued to Chu and Kronis, is considered the minimum NA necessary to trap and manipulate particles in three dimensions. It is a drawback of the apparatus described in U.S. Pat. No. 6,055,106 that each lens requires a relatively large amount of physical space to operate within and each lens must be maintained, cleaned and aligned. Those familiar with transfer lens systems will recognize that the greater the number of lenses in the system the more opportunity for misalignment and other maintenance problems. Accordingly, there has existed a need to reduce the number of lenses in a transfer lens system used for forming an array of optical traps. The present invention satisfies this need.
A common modality for monitoring the activity of an optical trap described in U.S. Pat. No. 6,055,106 is to place a beam splitter in the pathway of the laser beam and thereby yield an optical data-stream. One limitation of this modality is the detrimental effect on the optical data stream of noise. In the context of optical traps, noise refers to the interference with the imaging, measuring and/or viewing of the optical traps, their contents, or the surrounding region resulting from the presence in the system of un-diffracted focused beam of light or energy, light emanating from the optical traps, and light reflected or diffracted off a lens in a physical transfer lens system either due to imperfections in the lens, dust, dirt or due to misalignment. As taught in U.S. Pat. No. 6,055,106, one way to reduce noise is to direct the laser beam at an oblique angle relative to the diffractive element thereby urging the un-diffracted beam away from the objective lens. While useful for its intended purposes, the other sources of noise remain. There exists a need for reduction or elimination of the noise caused by the un-diffracted laser beam, scattered and reflected laser light off the components of a system producing an array of optical traps. The present invention also satisfies this need and other needs and provides related advantages.